Stator assembly for a brushless motor in a power tool

ABSTRACT

A power tool is provided including a housing and a motor housed inside the housing. The motor includes a stator assembly and a rotor pivotably arranged inside the stator assembly. The stator assembly includes a lamination stack defining poles extending radially towards a center of the lamination stack, pole teeth extending laterally with respect to ends of the poles, and stator slots formed between adjacent poles. The stator assembly also includes field windings wound around the poles. Electrically non-conductive tooth damper inserts are longitudinally arranged inside the stator slots between adjacent field windings, each tooth damper insert including a radial outer end in contact with an outer wall of the slot and a radial inner end arranged at an open end of the slot and engaging lateral edges of two opposing teeth within the slot.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of prior filed co-pending U.S. Provisional Application No. 61/660,335, filed Jun. 15, 2012, content of which is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

This disclosure relates to a power tool, and more particularly to noise reduction mechanism in a stator assembly for an electric brushless DC motor for a power tool.

BACKGROUND

The use of cordless power tools has increased dramatically in recent years. Cordless power tools provide the ease of a power assisted tool with the convenience of cordless operation. Conventionally, cordless tools have been driven by Permanent Magnet (PM) brushed motors that receive DC power from a battery assembly or converted AC power. The motor associated with a cordless tool has a direct impact on many of the operating characteristics of the tool, such as output torque, time duration of operation between charges and durability of the tool. The torque output relates to the capability of the power tool to operate under greater loads without stalling. The time duration of the power tool operation is strongly affected by the energy efficiency of the motor. Since, during some operating modes cordless tools are powered by battery modules that contain a limited amount of energy, the greater the energy efficiency of the motor, the longer the time duration that the tool can be operated. The durability of a power tool is affected by many factors, including the type of motor that is used to convert electrical power into mechanical power.

Brushed motors such as the PM brushed motors that are generally employed in power tool applications are susceptible to damaged brushes over time. The main mechanical characteristic that separates Permanent Magnet brushless motors from Permanent Magnet brushed motors is the method of commutation. In a PM brushed motor, commutation is achieved mechanically via a commutator and a brush system. Whereas, in a brushless DC motor, commutation is achieved electronically by controlling the flow of current to the stator windings. A brushless DC motor includes a rotor for providing rotational energy and a stator for supplying a magnetic field that drives the rotor. Comprising the rotor is a shaft supported by a bearing set on each end and encircled by a permanent magnet (PM) that generates a magnetic field. The stator core includes field windings around the rotor. Power devices such as MOSFETs are connected in series with each winding to enable power to be selectively applied. When power is applied to a winding, the resulting current in the winding generates a magnetic field that couples to the rotor. The magnetic field associated with the PM in the rotor assembly attempts to align itself with the stator generated magnetic field resulting in rotational movement of the rotor. A control circuit sequentially activates the individual stator coils so that the PM attached to the rotor continuously chases the advancing magnetic field generated by the stator windings. A set of sense magnets coupled to the PMs in the rotor assembly are sensed by a sensor, such as a Hall Effect sensor, to identify the current position of the rotor assembly. Proper timing of the commutation sequence is maintained by monitoring sensors mounted on the rotor shaft or detecting magnetic field peaks or nulls associated with the PM.

A brushless motor provides many advantages over conventional brushed motors. Conventional brushed motors are substantially less durable than brushless motors because of the wear and tear associated with the brushes. Also, since commutation is handled via a microcontroller, mechanical failures associated with the commutation are minimized and fail conditions are better managed and handled. Furthermore, brushed motors are less efficient than brushless motors due to the friction and the heat associated with the brushes and the commutator. It was found, however, that certain brushless motor designs created a resonating noise that is unpleasant to the end user when used in high-speed and/or high-power applications such as high speed power tools. What is needed is a mechanism to reduce noise causes by the motor.

SUMMARY

According to an embodiment of the invention, a power tool is provided. The power tool may be, for example, a drill or an impact driver, although other types of power tools may also be used. The power tool includes a housing and a motor, such as a brushless DC motor, housed inside the housing. The motor includes a stator assembly and a rotor pivotably arranged inside the stator.

According to an embodiment, the stator assembly includes a lamination stack defining poles extending radially towards a center of the lamination stack, pole teeth extending laterally with respect to ends of the poles, and stator slots formed between adjacent poles. The stator assembly also includes field windings wound around the poles; and electrically non-conductive tooth damper inserts longitudinally arranged inside the stator slots between adjacent field windings. In an embodiment, each tooth damper insert includes a radial outer end in contact with an outer wall of the slot and a radial inner end arranged at an open end of the slot and engaging lateral edges of two opposing teeth within the slot. According to an embodiment, the radial outer ends of the tooth damper inserts provide support against radial movement and vibration of lamination of the lamination stack, and the radial inner ends of the tooth damper inserts provide support for the poles and the pole teeth against rotational or radial movement and vibration.

According to an embodiment, the radial outer end of the tooth damper insert is in contact with an inner surface of the lamination stack defining the outer wall of the slot.

According to an embodiment, the stator assembly further comprises an end insulator arranged at a longitudinal end of the lamination stack. The end insulator may include slot walls protruding into slots of the lamination stack to define the stator slots. In an embodiment, the radial outer end of each tooth damper insert is in contact with an inner surface a corresponding slot wall of the end insulator defining the outer wall of the corresponding slot.

According to an embodiment, the radial outer end of the tooth damper insert is arcuate-shaped to follow a profile of the outer wall of the slot. According to an embodiment, the radial inner end includes two radial notches arranged to engage inner surfaces of the two opposing teeth of the slot. According to an embodiment, the tooth damper inserts are slidably arranged in the stator slots.

According to an embodiment, a ring element is provided and attached to the tooth damper inserts and mounted on an outer surface of the stator assembly. According to an embodiment, the radial inner end of the tooth damper inserts and the stator teeth are aligned along a circle.

According to an embodiment, the motor is a permanent magnet brushless direct current (DC) motor.

According to an embodiment of the invention, a permanent magnet brushless direct current (DC) electric motor having a stator assembly and a rotor pivotably arranged inside the stator assembly. The stator assembly includes the above-disclosed features, including electrically non-conductive tooth damper inserts longitudinally arranged inside the stator slots between adjacent field windings, each tooth damper insert including a radial outer end in contact with an outer wall of the slot and a radial inner end arranged at an open end of the slot and engaging lateral edges of two opposing teeth within the slot.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings described herein are for illustration purposes only and are not intended to limit the scope of this disclosure in any way:

FIG. 1 depicts a perspective cross-sectional view of a power tool, according to an embodiment of this disclosure;

FIGS. 2A and 2B depict perspectives expanded views of a brushless electric motor, according to an embodiment of this disclosure;

FIG. 2C depicts a perspective cross-sectional view of the brushless electric motor of FIGS. 2A and 2B, according to an embodiment of this disclosure;

FIGS. 3A-3C depict various configurations for connecting stator windings of a brushless motor;

FIG. 4 depicts an exemplary speed-torque diagram of a brushless motor with different stator winding configurations;

FIGS. 5A and 5B depict a motor stator connected to achieve a delta configuration, according to an embodiment of the invention;

FIG. 5C depicts the bus bar connection of a stator assembly, according to an embodiment of the invention;

FIGS. 5D-5H depict conductive plates of the bus bar of FIG. 5C and wire connections thereto, according to an embodiment of the invention;

FIG. 6 depicts a perspective view of a bus bar of a stator assembly, according to an alternative embodiment;

FIG. 7 depicts a perspective view of the bus bar of FIG. 6, according to an embodiment;

FIG. 8 depicts a partial cross-sectional view of a power tool and motor having the stator assembly of FIG. 6, according to an embodiment;

FIG. 9 depicts a perspective view of a stator assembly including tooth damper inserts, according to an embodiment;

FIG. 10 depicts a cross-sectional view of the stator assembly with the tooth damper inserts inserted between the stator teeth, according to an embodiment;

FIG. 11 depicts an expanded view of the stator assembly showing both the tooth damper inserts of FIG. 9 and the bus bar of FIG. 6;

FIG. 12 depicts a perspective view of a stator assembly having a ring of tooth damper inserts, according to an alternative embodiment;

FIG. 13 depicts a perspective view of the stator assembly of FIG. 12 with the ring of tooth damper inserts being fully inserted, according to an embodiment.

DESCRIPTION

With reference to the FIG. 1, a power tool 100 constructed in accordance with the teachings of the present disclosure is illustrated in a longitudinal cross-section view. The power tool 100 in the particular example provided may be a drill/driver, but it will be appreciated that the teachings of this disclosure is merely exemplary and the power tool of this invention could be a circular saw, a reciprocating saw, or any similar portable power tool constructed in accordance with the teachings of this disclosure. Moreover, the output of the power tool driven (at least partly) by a transmission constructed in accordance with the teachings of this disclosure need not be in a rotary direction.

The power tool shown in FIG. 1 may include a housing assembly 102, a motor assembly 104, a control module 104, a battery pack 108, an input unit (e.g., a variable speed trigger) 110, a transmission assembly 114, an output spindle (not shown), and a chuck (not shown) that can be coupled for rotation with the output spindle. The housing assembly 102 can include a housing 102 a and a gear case 102 b that can be removably coupled to the housing 102 a. The housing 102 a can define a housing body and a handle 112.

According to an embodiment, the motor 104 is received in the housing 102 a. The motor can be any type of motor and may be powered by an appropriate power source (electricity, pneumatic power, hydraulic power). In the particular example provided, the motor is a brushless DC electric motor and is powered by a battery pack 108. An input unit 110 is mounted in the handle 112 below the housing 102 a. The input unit 110 may be a variable speed trigger switch, although other input means such as a touch-sensor, a capacitive-sensor, a speed dial, etc. may also be utilized. In an embodiment, variable speed trigger switch may integrate the ON/OFF, Forward/Reverse, and variable-speed functionalities into a single unit and provide respective inputs of these functions to the control unit 106. The control unit 106, which is coupled to the input unit 110 as described further below, supplies the drive signals to the motor. In the exemplary embodiment of the invention, the control unit 106 is provided in the handle 112.

The brushless motor 104 depicted in FIG. 1 is commutated electronically by the control unit 106. The tool 100 is powered by a suitable power source such as the battery pack 108. It is envisioned, however, that the present disclosures can be applied to a power tool with an AC power source, which may further include an AC-to-DC converter to power to motor. Using the variable-speed input and other inputs from the input unit 110, the control unit 106 controls the amount of power supplied to the motor 104. In an exemplary embodiment, the control unit 106 controls the Pulse Width Modulation (PWM) duty cycle of the DC power supplied to the motor 104.

Referring now to FIGS. 2A and 2B, perspectives expanded views of the brushless electric motor 104 is depicted according to an embodiment of the invention. FIG. 2C depicts a cross-sectional view of the brushless motor 104. As shown in these figures, in an exemplary embodiment, the brushless motor 104 includes Hall board mount assembly 210, a stator assembly 230, a rotor assembly 250, and a ring gear mount 270.

The Hall board assembly includes a Hall board mount 212 and a Hall board 214. The Hall board 214 snaps onto the Hall board mount 212 via a plurality of pins 216, which may then be welded over the Hall board 214. The Hall board mount 212 includes a bearing support 218 that receives an end bearing 252 of the rotor assembly 250. Mounted on the Hall board 214 are one or more Hall Effect sensors 220 arranged around the circumference of the bearing support 218. The Hall board mount 212 further includes a Hall Effect Sensor interference 222 that is coupled to the control unit 106 to provide the control unit 106 with Hall Effect sense signals.

The stator assembly 230 includes a stator 240 having a plurality of stator windings 232 housed in a stator lamination stack 242. In a six-pole three-phase brushless electric motor, as shown in this exemplary embodiment, three stator windings 232 are provided within the lamination stack 242. Each stator winding 232 is distributed around the lamination stack 242 to form an even number of poles. In a six-pole stator, each stator winding 232 includes a pair of windings arranged at opposite ends of the lamination stack 242 to face each other. The stator windings 232 may be connected in a variety of configurations. Exemplary configurations include a series delta configuration, a parallel delta configuration, a series wye configuration, and a parallel wye configuration. The distinguishing characteristics of these configurations will be discussed later in detail. The stator assembly 230 further includes a bus bar 234 coupled to the control unit 106 to receive DC power from the control unit 106 to power the field windings 232. Using the bus bar 234 and based on the input from the Hall Effect sensors 218, the control unit 106 sequentially commutates the stator windings 232 to drive the rotor 254. In addition, the stator assembly 230 includes a baffle 236 coupled to the stator 240 via snaps or pins 238. The baffle 235 may include a protrusion 236 a at its low end to contain the wiring connections from the bus bar 234 to the stator windings 232. Alternatively, the baffle 235 may itself integrally include the bus bar 234 to input power from the control unit 106.

In an embodiment, the stator assembly 230 includes alignment features, i.e., pins 310 and receptacles 302, that mate with corresponding alignment features 304, 306 provided on the Hall board mount assembly 210 and ring gear mount 270.

FIGS. 3A-3C show different stator windings 232 connections used to achieve the series wye (“Y” shaped) (FIG. 3A), series delta (FIG. 3B), and parallel delta (FIG. 3C) configurations. A parallel wye configuration may also be achieved, although such configuration is not explicitly shown. The three stator windings in a six-pole brushless motor are typically designated as U-U₁; V-V₁; and W-W₁ windings, where each winding includes two poles (U and U₁, for example, designate two poles of the same winding). The wye configuration, sometimes called a star winding, connects all of the windings to a neutral (e.g., ground) point and power is applied to the remaining end of each winding. The delta configuration connects the three windings to each other in a triangle-like circuit, and power is applied at each of the connections. For a given motor, the delta configuration achieves higher speed (rpm) at lower torque, whereas the wye configuration achieves relatively higher torque at lower speed. The parallel delta configuration achieves the even higher speed at lower torque load. FIG. 4 depicts an exemplary speed-torque diagram of a brushless motor having these configurations.

In a typical off-the-shelf stator assembly for an electric brushless motor, the poles of each stator windings 232 (i.e., U and U₁, V and V₁, and W and W₁) are arranged opposite one another and are wound using a single wire during the manufacturing process. Specifically, the stator housing typically includes pre-routed wiring connections that connects terminals 2 (U) and 7 (U₁), terminal 4 (V) and 9 (V₁), and terminals 6 (W) and 11 (W₁) around or adjacent to the stator lamination stack 242 (See FIG. 5A). The remaining terminals may then be wired to achieve the desired configuration, i.e., delta or wye, in series or in parallel.

Conventionally, in a six-pole motor, three adjacent poles are designated as U, V, and W, opposite the corresponding U₁, V₁, and W₁ poles of the same winding 232. FIG. 5A depicts the brushless motor 104 with this arrangement. A challenge with this arrangement, however, is that terminals 1 (U) and 12 (W₁), terminals 5 (W) and 10 (V₁) and terminals 3 (V) and 8 (U₁) must be wired together to obtain the delta configuration. It is easy to wire terminals 1 and 12 to each other, as they are located adjacent to one another. However, connecting terminals 5 and 10 and terminals 3 and 8 require wiring around the circumference of the stator 240. Furthermore, some conventional designs utilize a printed circuit board attached to the stator to facilitate the connections between the stator terminals, but the copper tracks of the printed circuit board are typically insufficient in handling large amounts of current in heavy duty power tool applications, such as drills or other high torque power tools.

In order to overcome this challenge, according to an alternative embodiment of the invention shown in FIG. 5B, the poles of the stator windings are designated such that the terminals required for wiring a delta connection are arranged adjacent to one another. For example, in an exemplary embodiment, the designation of the stator windings poles V and V₁ are switched such that terminals 5 and 10 as well as terminals 3 and 8 fall adjacent to one another. Accordingly, the terminals can be connected easily without the need for extra wiring through the center or around the circumference of the stator 240. The stator windings 232 can be comprised of one continuous coil tapped at three connection points 502 a, 502 b, 502 c for connecting the stator windings 232 to the bus bar 234. This arrangement significantly simplifies the motor winding process.

FIGS. 5C-5H depict the details of the bus bar 234 and the wiring of the stator assembly 230, according to an embodiment of the invention.

As shown in FIG. 5C, the stator 240 includes the lamination stack 242 and end insulators 550, 552. The bus bar 234 includes at least three input terminals 502 corresponding to each of the stator windings U, V and W. In an exemplary embodiment, the input terminals 502 comprise conductive plates 504 separated by insulating channels 506. The insulating channels 506, in an embodiment, are formed by a longitudinal extension portion extending over the outer surface of the lamination stack 242, and retaining walls extending vertically from the extension portion. The extension portions and the retaining walls are formed with the end insulators 550, 552 and are mated together over the outer surface of the lamination stack 242 to form the insulating channels 506. The bus bar 234 is arranged on an outer periphery of the stator lamination stack 242 and the conductive plates 502 are longitudinally arranged along the outer periphery of the stator lamination stack 242. The conductive plates 504 may be made of, for example, brass material or other conductive metal. As shown in FIG. 5D, each conductive plate 504 may include one or more barb features 512, 514 for attaching the conductive plates 504 inside the insulating channels 506. Further, as shown in FIGS. 5C and 5E, each conductive plate 504 may include a first planar portion arranged inside the insulating channels 506 and a second arranged at an angle from the first planar portion. The conductive plates include hooks 516 for routing wires from the stator windings to the conductive plates 504 extending from the first planar portions. The conductive plate 504 may also include hooks 518 for accommodating the wires from the control unit 106 to the conductive plates 504 extending from the second planar portions.

As shown in FIG. 5F, the barb features 514 of the conductive plates 504 snap into corresponding receiving slots 524 inside the insulating channels 506. The insulating channels 506 are shown in this figure without the walls separating the channels 506. Further, as shown in cross-sectional view of FIG. 5G, the barb features 512 engage protrusions 522 of the insulating channels 506 to lock the conductive plates 504 within the insulating channels 504. Wires 530 from the control unit 106 may be soldered or attached by other means inside the hooks 518. Similarly, wires 532 from the stator windings 232 may be soldered or otherwise attached inside the hooks 516. FIG. 5H depicts an expanded view of the stator assembly 230 including the wires 532 leading from the stator windings 232 and through the insulating channels 506 into the hooks 516.

The above-described embodiment of the bus bar 234 provides several advantages. First, since the terminals (i.e., conductive plates 504) are provided on the outer surface of the stator lamination stack, no additional space is taken up longitudinally. This reduces the overall length of the stator assembly. Also, the insulating channels that retain the terminals are molded as a part of the end insulators 550, 552, which significantly eases the manufacturing process.

In the above-discussed embodiment, during the course of motor manufacturing, the ends of the stator magnet wires are stripped of wire insulation and paired together. The pairs of leads are then received inside corresponding hooks 516 of conductive plates 504, and the hooks are crimped and soldered to the wire leads. The power inputs from the control unit and power source are similarly connected to the other corresponding hooks 518 of the conductive plates 504. While this arrangement may be desirable in some applications, the stripping and crimping steps may pose challenges during the motor manufacturing process.

FIGS. 6-8 depict an alternative and improved bus bar 334, according to an alternative embodiment of the invention. FIG. 6 shows a stator assembly 340 including the bus bar 334, according to this embodiment. FIG. 7 shows a perspective view of the bus bar 334. FIG. 8 illustrates a cross-sectional view of the stator assembly 340 including the bus bar 334 inside a power tool housing.

In this embodiment, the bus bar 334 is arranged on an outer surface of the stator lamination stack 242. The bus bar 334 may extend fully or partially along the outer surface (i.e., outer periphery) of the stator lamination stack 242. The bus bar 334 includes three terminals (also referred to as conducive terminals or conductive plates) 604 arranged longitudinally along the outer surface of the lamination stack 242. In an embodiment, the conductive plates 604 are mounted on a non-conductive mount 602. The conductive plates 604 are separated and insulated from each other via the mount 602. The mount 602 is mounted on the outer surface of the lamination stack 242. The end insulators 650 and 652, which are arranged at the longitudinal ends of the lamination stack 242, are provided with retaining walls 651 and 653, respectively, which mate together over the outer surface of the lamination stack 242 around the mount 602 to retain the mount 602 over the outer surface of the lamination stack 242.

Conductive plates 604 in this embodiment include connection tabs 618 arranged at a first longitudinal distal end in the proximity of the end insulator 650, and tangs 616 arranged at a second longitudinal distal end in the proximity of the end insulator 652. The tangs 616 fold back over a main surface of the conductive plates 604 in the longitudinal direction of the conductive plates 604. The stator magnet wire 620, which is wound around the lamination stack slots to form stator coils, are routed over the end insulator 652 between the coils to connect the coils in wye or delta configurations. The end insulator 652 may be provided with routing features for routing and positioning the wire 620. In an embodiment, the magnet wire 620 may be wrapped around the tangs 616 at various points, e.g., either at the wire leads or at mid points, to facilitate the desired winding configuration. For example, for a delta connection (see FIG. 3B), the magnet wire 620 may be first wrapped around a first tang 616 at a lead end, routed and wound to form coils W and W1, wrapped around a second tang 616, routed and wound to form coils U and U1, wrapped round a third tang 616, routed and wound to form coils V and V1, and terminated at the first tang 616. The fusing process burns a portion of the magnet wire insulation material that is in contact with the tangs 616. The fusing step thus eliminates the need for stripping the contact portion of the magnet wires. In an embodiment, the tangs 616 may be pressed over the magnet wire contact portion.

Connection tabs 618 project outwardly from the bus bar 334 and the stator lamination stack 242. Each tab 618 may include a through-hole therein. The motor wires (not shown) received from the control unit, which carry electric power to the motor field windings, may be inserted into the through-holes of the corresponding tabs 618 and soldered. Alternatively, the motor wires may be welded to the connection tabs 618. In yet another embodiment, additional terminals 660, as shown in FIG. 8, may be welded to the connection tabs 618.

The aforementioned embodiment offers several advantages. For example, the tang 616 geometry of the bus bar 334 may be designed to accommodate any amount of wire 620 leads and wire diameter. Also, the bus bar 334 may be designed to accommodate any lamination stack 242 length. The fusing of the stator magnet wires and attaching the motor wires also becomes easier using this embodiment. It must be noted that while the mount 602 is shown as a separate piece, the end insulators 650 and 652 may be provided with features to integrally form the mount 602. Also, the end insulators 650 and 652 may be provided with retaining features to support and retain the conductive plates 604 at various locations around the outer periphery of the stator assembly, e.g., at 120 degree angles.

Another aspect of this disclosure is discussed herein with continued reference to FIGS. 2A and 2B, and further in reference to FIGS. 9-12. It was found that many brushless motors platform, such as, for example, those using the stator assembly depicted in FIGS. 2A and 2B, develop a resonating frequency that is unpleasant to the end user when used in high-speed and/or high-power applications such as high speed power tools. The inventors of this application found that this noise is attributable to the stator laminations flexing and distorting at high speed. Specifically, as the rotor 250 rotates inside the stator assembly 230, the magnetic force of the rotor 250 magnets cause a small amount of vibration in the stator lamination poles. It was initially believed that the noise is generated by small movements of the individual laminations with respect to one another. However, it was found that reinforcing the laminations with respect to one another via adhesive or other means does not reduce the noise. The inventor then found that the lateral vibration of the poles and the pole teeth is primarily responsible for the noise. One solution is to increase the thickness of the lamination stack behind each pole. The reinforced metal behind each pole reduces the amount of unwanted vibration. However, increasing the thickness of the lamination stack increases the overall motor size, which is undesirable in many power tool applications.

In order to reduce the noise from the lamination stack poles and pole teeth, according to an embodiment on this disclosure, non-conductive tooth damper inserts may be provided inside the lamination stack slots to provide further support for the laminations teeth, as shown in FIGS. 9-13. In one embodiment, as shown in FIGS. 9 and 10, the tooth damper inserts 920 are provided within each slot 904 of the stator assembly 340 extending in a direction of the center of the stator assembly 340. FIG. 9 illustrates a perspective view of the stator assembly 340, with the tooth damper inserts 920 pulled out of the slots 904 for a better view. FIG. 10 illustrates a cross-sectional view of the stator assembly 340, with the tooth damper inserts 920 inserted in to the slots 904.

In an embodiment, each tooth damper insert 920 may include two radial end portions. A radial outer end 924 engages an outer wall of the slot 904 defined by an inner surface of the stator assembly 340, i.e., the inner surface of the lamination stack 242 and/or the slot wall portions of the end insulators 650 and 652 at the back of the slot 904. The radial outer end 924 may have an arcuate shape following a profile of the outer wall of the slot 904. The radial outer end 924 provides support for the laminations against radial movement and vibration.

In an embodiment, a radial inner end of the tooth damper insert 920 is arranged at an open end of the slot 904 formed between adjacent stator teeth 906 and engages lateral edges of the opposing teeth 906 that define the open end of the slot 904. In an embodiment, the second end portion includes two side projections (i.e., notches) 922 that engage back edges of the teeth 906. The second end portion of the tooth damper insert helps support the laminations teeth against rotational and/or radial movement and vibration. In an embodiment, the tooth damper inserts 920 are inserted into the slots 904 after the stator assembly 340 is fully wound. In an embodiment, the radial inner end of the tooth damper inserts 920 and the stator teeth 906 are aligned along a circle.

FIG. 11 depicts an expanded view of the stator assembly 340, including the tooth damper inserts 920, stator field windings 902, magnet wires 620, stator lamination stack 242, end insulators 650 and 652, and the bus bar 334, according to an embodiment of the invention. The end insulators 650 and 652 are shown with walls 251 and 253 that mate together to securely hold the bus bar 334. The end insulators 650 and 652 are also shown with slot walls 650 a and 652 a that tightly penetrate into the slots of the lamination stack 242. The slot walls 650 a and 652 a support to the tooth damper inserts 920 inside the slots 904. In an embodiment, each of the slot walls 650 a and 652 a extends within the interior of the slot 604 from one tooth 906 to another. In an embodiment, the lateral edges of each slot wall 650 a and 652 a are positioned in the proximity of the inner edges of the teeth 906 so as to be in contact with (or close to) notches 922 of the tooth damper inserts 920. This allows the tooth damper inserts 920 notches 922 to engage the metallic edges of the teeth 906.

FIGS. 12 and 13 depict teeth damper inserts pre-assembled into a ring 950, according to an embodiment. In this embodiment, the tooth damper inserts are pre-assembled into a ring 950 prior to insertion into the lamination stack slots. The ring 950 may include an outer ring 954 and an inner ring 956, with an upper surface of the tooth damper inserts extending from the outer ring 954 to the inner ring 952. The ring 950 includes through-holes corresponding to the stator coils 902 formed between the tooth damper inserts. The ring 950 may also include retention features 958 corresponding to snaps 915 provided on the end insulator 650. The ring 950 in this embodiment provides support for the tooth damper inserts. Additionally, the integrated ring 950 enables insertion of the tooth damper inserts in a single step, thus improving the assembly process.

The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the scope of the invention. 

1. A power tool comprising a housing and a motor housed inside the housing, the motor having a stator assembly and a rotor pivotably arranged inside the stator assembly, the stator assembly comprising: a lamination stack defining a plurality poles extending radially towards a center of the lamination stack, a plurality of pole teeth extending laterally with respect to ends of the poles, and a plurality of stator slots formed between adjacent poles; a plurality of field windings wound around the poles; and a plurality of electrically non-conductive tooth damper inserts longitudinally arranged inside the stator slots between adjacent field windings, each tooth damper insert including a radial outer end in contact with an outer wall of the slot and a radial inner end arranged at an open end of the slot and engaging lateral edges of two opposing teeth within the slot.
 2. The power tool of claim 1, wherein the radial outer end of the tooth damper insert is in contact with an inner surface of the lamination stack defining the outer wall of the slot.
 3. The power tool of claim 1, wherein the stator assembly further comprises an end insulator arranged at a longitudinal end of the lamination stack.
 4. The power tool of claim 3, wherein the end insulator comprises a plurality of slot walls protruding into slots of the lamination stack to define the plurality of stator slots, and the radial outer end of each tooth damper insert is in contact with an inner surface a corresponding slot wall of the end insulator defining the outer wall of the corresponding slot.
 5. The power tool of claim 1, wherein the radial outer end of the tooth damper insert is arcuate-shaped to follow a profile of the outer wall of the slot.
 6. The power tool of claim 1, wherein the radial inner end comprises two radial notches arranged to engage inner surfaces of the two opposing teeth of the slot.
 7. The power tool of claim 1, wherein the tooth damper inserts are slidably arranged in the stator slots.
 8. The power tool of claim 1, further comprising a ring element attached to the plurality of tooth damper inserts and mounted on an outer surface of the stator assembly.
 9. The power tool of claim 1, wherein the radial outer ends of the tooth damper inserts provide support against radial movement and vibration of lamination of the lamination stack, and the radial inner ends of the tooth damper inserts provide support for the poles and the pole teeth against rotational or radial movement and vibration.
 10. The power tool of claim 1, wherein the radial inner end of the plurality of tooth damper inserts and the plurality of stator teeth are aligned along a circle.
 11. The power tool of claim 1, wherein the motor is a permanent magnet brushless direct current (DC) motor.
 12. A permanent magnet brushless direct current (DC) electric motor having a stator assembly and a rotor pivotably arranged inside the stator assembly, the stator assembly comprising: a lamination stack defining a plurality poles extending radially towards a center of the lamination stack, a plurality of pole teeth extending laterally with respect to ends of the poles, and a plurality of stator slots formed between adjacent poles; a plurality of field windings wound around the poles; and a plurality of electrically non-conductive tooth damper inserts longitudinally arranged inside the stator slots between adjacent field windings, each tooth damper insert including a radial outer end in contact with an outer wall of the slot and a radial inner end arranged at an open end of the slot and engaging lateral edges of two opposing teeth within the slot.
 13. The electric motor of claim 12, wherein the radial outer end of the tooth damper insert is in contact with an inner surface of the lamination stack defining the outer wall of the slot.
 14. The electric motor of claim 12, wherein the stator assembly further comprises an end insulator arranged at a longitudinal end of the lamination stack.
 15. The electric motor of claim 14, wherein the end insulator comprises a plurality of slot walls protruding into slots of the lamination stack to define the plurality of stator slots, and the radial outer end of each tooth damper insert is in contact with an inner surface a corresponding slot wall of the end insulator defining the outer wall of the corresponding slot.
 16. The electric motor of claim 12, wherein the radial outer end of the tooth damper insert is arcuate-shaped to follow a profile of the outer wall of the slot.
 17. The electric motor of claim 12, wherein the radial inner end comprises two radial notches arranged to engage inner surfaces of the two opposing teeth of the slot.
 18. The electric motor of claim 12, wherein the tooth damper inserts are slidably arranged in the stator slots.
 19. The electric motor of claim 12, further comprising a ring element attached to the plurality of tooth damper inserts and mounted on an outer surface of the stator assembly.
 20. The electric motor of claim 12, wherein the radial outer ends of the tooth damper inserts provide support against radial movement and vibration of lamination of the lamination stack, and the radial inner ends of the tooth damper inserts provide support for the poles and the pole teeth against rotational or radial movement and vibration. 